Method and system for detecting signal sources in a surveillance space

ABSTRACT

A respective electromagnetic parameter and spatial disposition of an unknown number of signal sources in a surveillance space simultaneously bombarded by multiple signals are determined by receiving multiple signals at each of a plurality of widebeam, wideband antennas equally spaced apart in a linear array. Respective antenna signals are simultaneously sampled to generate a two-dimensional array of values. A two-dimensional Fourier transform is computed whose peaks satisfy one or more predetermined criteria, each peak being indicative of a signal source in the surveillance space, whereby the location of the peak in the Fourier transform Fjk indicates the frequency and the azimuth of the respective signal source and the amplitude of the peak indicates the amplitude of the signal source. When implemented using two mutually perpendicular arrays of receiving antennas, an additional Fourier transform of the two-dimensional Fourier transform generates, for each identified emitter, independent azimuth and elevation angles.

This is a Continuation of application Ser. No. 12/514,523 filed May 12,2009, which is a National Phase of Application No. PCT/IL2007/001329filed Oct. 31, 2007. The disclosure of the prior applications are herebyincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

This invention relates to surveillance systems, and more specifically tomethods for determining parameters of a source of electromagneticradiation.

BACKGROUND OF THE INVENTION

Surveillance systems are used for continuous detection and tracking ofsignals emitted by a signal source in a region of space undersurveillance. The signals are received by an antenna array directedtowards the space under surveillance and processed to determine signalparameters such as frequency and azimuth direction. The signal sourcemay be an active stationary or moving transmitter that transmits EMradiation, such as radio transmitter, wireless telephone and so on. Itmay also be a passive source corresponding to a reflected signal, suchas a signal that originates from an active transmitter and is picked upand subsequently reflected by a radar antenna or any other radiationsource. So far as the present invention is concerned, it is immaterialwhether the signal source is active or passive. The signals received bythe receiving antenna array are processed so as to distinguish genuinesignals from noise and to determine the frequency, amplitude anddirection of each detected signal.

A problem with hitherto proposed detection systems is that detection ofweak signals is difficult, thus imposing severe restrictions on themaximum distance from the antenna array for which signal detection ispossible. Moreover, known systems are unable to distinguish betweensignals of identical frequency originating from signal sources that arespatial disposed in different directions.

SUMMARY OF THE INVENTION

The present invention provides a method and system for determiningelectro-magnetic properties of a signal source such as frequency andazimuth angle detected in a space under surveillance. The method andsystem according to the invention may be used when neither the number ofsignal sources, nor the frequency and direction of the signal sourcesare known a priori to the system.

In accordance with a first aspect of the invention there is provided amethod for determining a respective electromagnetic parameter andspatial disposition of one or more signal sources in a surveillancespace simultaneously bombarded by multiple signals, the methodcomprising:

receiving said multiple signals at each of a plurality of widebeam,wideband antennas that are spaced apart in a linear array by equalmutual spacings;

simultaneously sampling the respective signals A_(i,n) of each of theantennas at a sampling rate to generate a two-dimensional array ofsampled values S_(i,n), where S_(i,n) is the n-th sample of the signalA_(i,n) received at an antenna i;

calculating a two-dimensional Fourier transform F_(jk) of the arrayS_(i,n); and

identifying peaks in the Fourier transform satisfying one or morepredetermined criteria, a peak satisfying the predetermined criteriabeing indicative of a signal source in the surveillance space, wherebythe location of the peak in said two-dimensional Fourier transformF_(jk) indicates the frequency and the azimuth of the respective signalsource and the amplitude of the peak indicates the amplitude of thesignal source.

In accordance with a second aspect of the invention there is provided asystem for determining a respective electromagnetic parameter andspatial disposition of one or more signal sources in a surveillancespace simultaneously bombarded by multiple signals, the systemcomprising:

an antenna array having a plurality of spaced apart widebeam, widebandantennas that are spaced apart in a linear array by equal mutualspacings and are configured to receive said multiple signals; and

a processor coupled to the antenna array and being configured to:

sample the respective signals S_(i,n) received by each of the antennasat a sampling rate to generate a two-dimensional array of sampled valuesS_(i,n), where S_(i,n) is the n-th sample of the signal received at adetector i;

calculate a two-dimensional Fourier transform F_(jk) of the arrayS_(i,n); and

identify peaks in the Fourier transform satisfying one or morepredetermined criteria, a peak satisfying the predetermined criteriabeing indicative of a signal source in the surveillance space, wherebythe location of the peak in said two-dimensional Fourier transformF_(jk) indicates the frequency and the azimuth of the respective signalsource and the amplitude of the peak indicates the amplitude of thesignal source.

The system of the invention comprises an array of antenna elements.Signal sources in the surveillance space radiate electromagnetic (EM)signals towards the receiving antenna array which collects the radiatedsignals. The frequency f of the EM radiation may or may not be known tothe system. The antenna array must, of course, be capable of receivingthe signals and to this end must be tuned to a frequency band in whichthe signal sources are located and must be adapted to receive a signalover a wide angle of view that contains all the signal sources. Thedistance traveled by the EM radiation from the object to the antennaarray is in general different for each receiving antenna in the array.The signals arriving at each of the receiving antennas are thus out ofphase from each other, the phase difference being a function of theincremental distance that each signal travels before being received. Thesignals are sampled to yield a two dimensional array of sampled valuesthat is input to an azimuth determination processing stage. The azimuthdetermination processing involves calculating a two dimensional Fouriertransform of the input array. The Fourier transform, has one index (or“bin number”) j that is a function of the frequency f of the EMradiation, and various parameters of the system. The other bin number ofthe Fourier transform, k, is a function of the frequency f, the azimuthangle θ and parameters of the system.

The Fourier transform is scanned for peaks satisfying predeterminedcriteria in order to identify signals in the surveillance space and tosegregate these signals from clutter. For each received signal, if thefrequency f of its EM radiation is unknown to the system, the frequencyf is determined from the bin number j of the peak. The azimuth angle θof the received signal is then determined from the bin number k of thepeak and the frequency f.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, some embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying diawings,in which:

FIG. 1 shows a block diagram of a surveillance system configured todetermine the azimuth angle and frequency of a signal source inaccordance with an embodiment of the invention;

FIG. 2 shows a method for determining the azimuth angle and frequency ofa signal source in accordance with an embodiment of the invention;

FIG. 3 shows a two-dimensional time-distance plot obtained by oneembodiment of the method of the invention; and

FIG. 4 is a flow diagram showing the principal operations carried out bya surveillance system for determining the azimuth angle and frequency ofa signal source in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention provides a system and method for determining theazimuth angle and signal frequency of a signal source that emits asignal, which is detected in a space under surveillance. FIG. 1 is ablock diagram schematically showing the hardware components and signalprocessing stages of a detection system 20 in accordance with oneembodiment of the invention. The system 20 comprises an antenna array 21consisting of a plurality of wideband antenna elements A. Four antennaelements A₀ to A₃ are shown in FIG. 1. This is by way of example only,and although the invention may be carried out using any number ofreceiving antennas greater than 1, the greater the number of receivingantennas in the array 21, the greater is the accuracy and sensitivity ofthe detection and consequently of the azimuth determination. Signalsources in the surveillance space, such as transmitters 22 to 25 (shownin FIG. 2) radiate electromagnetic (EM) pulses towards the receivingantenna array 21 which collects the received data.

FIG. 2 shows schematically an arrangement of the antenna array 21 inwhich adjacent receiving antennas are aligned along an axis 30 andseparated by a fixed distance d. The distance from the array 21 to asignal source detected in the surveillance space, such as thetransmitters 22 to 25 is sufficiently large, in comparison to the lengthof the array 21 that the respective rays R from each antenna in thearray to the transmitters are essentially parallel, and thus determinethe same azimuth angle θ with the axis 30.

The signal that is collected in the i-th receiver A; of the receivingantenna array 21 during the n-th sample may be modeled as:S _(i,n) =A cos [2πf(ndt+iΔt)+φ]+N_(i,n)  (1)where:

t=time,

A=amplitude,

f=frequency of the EM radiation,

dt=time delay of the signal between samples,

Δt=time delay of signals from adjacent antennas in the array, and

N_(i,n)=noise in i-th antenna during the n-th sample.

The signals S_(i,) have respective time delays iΔt owing to the factthat the distance that each signal S_(i,n) travels along the respectiveray 32 from the transmitter to the antenna A_(i) is different for eachsignal. In FIG. 2 it is seen that the distance traveled by the signalS_(i) along the ray R_(i) exceeds the distance traveled by the signal S₀along the ray R₀ by an amount Δt given by:

$\begin{matrix}{{\Delta\; t} = {\frac{d}{c}{\cos(\theta)}}} & (2)\end{matrix}$where c is the velocity of the propagation of the signals and d is thefixed distance between antennas.

Referring again to FIG. 1, each antenna A receives its respective signalS that is provided to its respective receiver channel RC. The signalsS_(i,n) are sampled by the receivers RC at a known sampling rate, toyield a two dimensional array of sampled values:S=A cos 2πf[(ndt+iΔt)+φ]+N _(i)  (3)where n is the sample number obtained using the sampling rate. For afixed value of n, the sequence S_(i,n) is a sampling of the signalemitted by the respective signal source 22 to 25 at a sampling rateequal to 1/dt.

For m simultaneous signals, this gives:

$\begin{matrix}{{S_{i,n} = {\sum\limits_{m}\left\lbrack {{A_{m}{\cos\left\lbrack {{2\pi\;{f_{m}\left( {{ndt} + {i\;\Delta\; t_{m}}} \right)}} + \varphi_{m}} \right\rbrack}} + N_{i,n}} \right\rbrack}}\;} & (4)\end{matrix}$

In one embodiment, the values of S_(i,n) are binarized by settingS_(i,n)=1 when S_(i,n)>0 and S_(i,n)=−1 when S_(i,n)≦0. Binarizing thesignals simplifies the hardware and software since a particular benefitof the invention over hitherto proposed approaches is that the inventionis able to detect weak signals, which individually are indistinguishablefrom noise. Any benefit in attempting to assign a multibit value to asignal that is of such small magnitude that it is indistinguishable fromnoise is insignificant and generally offset by the greater simplicity inthe hardware and processing that can be achieved by binarizing thesignals. On the other hand, when the dynamic range of simultaneousmulti-signals is required, multibit processing may be preferable usingfewer antennas so as to reduce processing complexity.

The two dimensional array S_(i,n) is input to an azimuth determinationprocessing stage 11. The azimuth determination processing involvescalculating a two dimensional Fourier transform of the array S_(i,n).The Fourier transform, referred to herein as a “frequency-azimuth plot”,is a two-dimensional array F_(jk). j is a bin number satisfying:

$\begin{matrix}{f = \frac{j \cdot {clock}}{N}} & (5)\end{matrix}$where f is the possibly unknown frequency of the EM radiation and N isthe number of bins of this dimension. k is a bin number satisfying:

$\begin{matrix}{f = \frac{k \cdot \left( {1/{dt}} \right)}{N^{\prime}}} & (6)\end{matrix}$where N′ is the number of bins of this dimension.

Substituting Equation (2) into Equation (6) yields:

$\begin{matrix}{f = \frac{k \cdot c}{{N^{\prime} \cdot d}\;\cos\;\theta}} & (7)\end{matrix}$or equivalently,

$\begin{matrix}{\theta = {{Arc}\;{\cos\left( \frac{k \cdot c}{N^{\prime} \cdot d \cdot f} \right)}}} & (8)\end{matrix}$

The array F_(jk) is scanned in the azimuth determination processingstage 11 for peaks satisfying one or more predetermined criteria inorder to identify signals emitted by the signal sources 22 to 25 in thesurveillance space and to segregate these signals from clutter. Thepredetermined criteria may include, for example, peak amplitude above apredetermined threshold. The frequency f of each identified signalsource is determined from the bin number j of the peak using Equation(5). This produces a value of f whose accuracy is determined by thenumber of bins N. The accuracy of the frequency determination can oftenbe improved by using phase data, as is known in the art of Fourieranalysis. In order to deal with the problem of frequency ambiguity, themethod of the invention may be performed at least twice using differentvalues of “clock” and/or by using an RF filter. Once the frequency isknown, the azimuth angle of the signal source is then determined fromthe bin number k of the peak using Equation (7). The output 26 of theprocessing stage 11 includes the azimuth angle for each detected signaland optionally the frequency of each detected signal.

FIG. 3 shows a two-dimensional frequency-azimuth plot 35 obtained onreal data by the method of the invention. The data were collected usinga linear array of 128 receivers. The plot was obtained using 512 signalsamples. The plot 35 has two peaks 36 and 37 corresponding to twodifferent signal sources in the surveillance area. The peak 36 reveals asignal having a frequency of f₁ and an azimuth angle of 30°. This signalwas detected with a signal to noise ration of −18 db. The peak 37reveals a signal having a frequency of f₂ and an azimuth angle of 50°.This signal was detected with a signal to noise ratio of −20 db.Increasing the number of detectors in the array enhances sensitivity.

FIG. 4 is a flow diagram summarizing the principal operations carriedout in accordance with an embodiment of the invention for determiningthe azimuth angle and frequency of a signal source as described abovewith particular reference to FIGS. 1 to 3.

It should be noted that the system of the invention may be implementedusing more than one array of receiving antennas. For example, theinvention may be implemented using two mutually perpendicular arrays ofreceiving antennas. An additional Fourier transform of thetwo-dimensional Fourier transform F_(jk) then generates, for eachidentified emitter, two independent angles (azimuth and elevation) thattogether define a position vector of the signal source.

It will also be appreciated that the signal source need not be an activetransmitter but could, for example, be a reflected signal.

It will also be appreciated that since the signals detected by thedetector array according to the invention are spatially separated intwo-dimensional space, signals having identical frequencies but spacedapart in two-dimensional space will be discretely detected.

Also, once the frequency and direction of a transmitter have beendetermined using FFT, Discrete Fourier Transform may be employed totrack the detected transmitter thus saving processing time.

It will also be understood that the system according to the inventionmay be a suitably programmed computer. Likewise, the inventioncontemplates a computer program being readable by a computer forexecuting the method of the invention. The invention furthercontemplates a machine-readable memory tangibly embodying a program ofinstructions executable by the machine for executing the method of theinvention.

1. A method for determining a respective electromagnetic parameter and spatial disposition of one or more signal sources in a surveillance space simultaneously bombarded by multiple signals, the method comprising: receiving said multiple signals at each of a plurality of antennas that are spaced apart in a linear array by predefined spacings; simultaneously sampling the respective signals of each of the antennas at a predetermined sampling rate to generate a two-dimensional array of sampled values S_(i,n), where S_(i,n) is the n-th sample of the signal received at an antenna i; calculating a two-dimensional Fourier transform F_(jk) of the array of sampled values S_(i,n); and identifying peaks in the Fourier transform satisfying one or more predetermined criteria, wherein an identified peak satisfying the predetermined criteria is indicative of a signal source in the surveillance space, and wherein a location of the peak in the Fourier transform F_(jk) indicates a frequency and an azimuth of the respective signal source and an amplitude of the peak indicates an amplitude of the signal source; and for the identified peak, calculating the frequency of the respective signal source using an algebraic involving the predetermined sampling rate.
 2. The method according to claim 1, wherein: said receiving comprises receiving said multiple signals at each of a plurality of widebeam, wideband antennas that are spaced apart in the linear array by equal mutual spacings.
 3. The method according to claim 2, including calculating the frequency of radiation emitted by the signal source indicated by an identified peak satisfying the one or more predetermined criteria, using an algebraic involving the predetermined sampling rate.
 4. The method according to claim 3, including calculating the frequency of the radiation emitted by the signal source indicated by the identified peak using the algebraic expression ${f = \frac{j \cdot {clock}}{N}},$ where f is the frequency of the radiation, j is a bin number of the peak and N is a number of bins.
 5. The method according to claim 2, including calculating an azimuth angle of the signal source indicated by an identified peak satisfying the one or more predetermined criteria.
 6. The method according to claim 5, wherein the azimuth angle of the signal source is calculated using an algebraic expression involving the frequency f of the radiation emitted by the signal source.
 7. The method according to claim 6, wherein the azimuth angle θof the signal source indicated by the peak is calculated using the algebraic expression ${\theta = {{Arc}\;{\cos\left( \frac{k \cdot c}{N^{\prime} \cdot d \cdot f} \right)}}},$ where c is the velocity of the radiation, k is a bin number of the peak and N′is a number of bins.
 8. The method according to claim 2, wherein the values of S_(i,n) , are set to 1 when S_(i,n)>0 and to −1 when S_(i,n)≦0.
 9. The method according to claim 2, wherein the multiple signals are received by a two-dimensional array of antennas such that along each of two mutually perpendicular axes respective ones of said antennas are spaced apart in a linear array by equal mutual spacings; the method further including: computing an additional Fourier transform of said two-dimensional Fourier transform F_(jk) so as to compute a three-dimensional Fourier transform that also provides elevation of each respective signal source.
 10. A system for determining a respective electromagnetic parameter and spatial disposition of one or more signal sources in a surveillance space simultaneously bombarded by multiple signals, the system comprising: an antenna array having a plurality of spaced apart antennas that are spaced apart in a linear array by predefined spacings and are configured to receive said multiple signals; and a processor coupled to the antenna array and being configured to: sample the respective signals received by each of the antennas at a sampling rate to generate a two-dimensional array of sampled values S_(i,n), where S_(i,n) is the n-th sample of the signal received at a detector i; calculate a two-dimensional Fourier transform F_(jk) of the array S_(i,n) ; and identify peaks in the Fourier transform satisfying one or more predetermined criteria, a peak satisfying the predetermined criteria being indicative of a signal source in the surveillance space, whereby a location of the peak in said two-dimensional Fourier transform F_(jk) indicates a frequency and an azimuth of the respective signal source and an amplitude of the peak indicates an amplitude of the signal source, wherein the processor is configured to calculate, for an identified peak satisfying the one or more predetermined criteria, the frequency of the respective signal source indicated by the peak using an algebraic involving the sampling rate.
 11. The system according to claim 10, wherein the antennas are arranged in a linear array having a uniform in spacing d between adjacent detectors, and wherein the processor is configured to calculate the frequency of radiation emitted by the signal source indicated by the peak using the algebraic expression ${f = \frac{j \cdot {clock}}{N}},$ where f is the frequency of the radiation, j is a bin number of the peak and N is a number of bins.
 12. The system according to claim 10, wherein the processor is further configured to calculate, for an identified peak satisfying the one or more predetermined criteria, an azimuth angle of the signal source indicated by the peak.
 13. The system according to claim 12, wherein the processor is configured to calculate the azimuth angle of the signal source using an algebraic expression involving the frequency f of the radiation emitted by the signal source.
 14. The system according to claim 13, wherein the processor is configured to calculate the azimuth angle θof the signal source indicated by the peak using the algebraic expression ${\theta = {{Arc}\;{\cos\left( \frac{k \cdot c}{N^{\prime} \cdot d \cdot f} \right)}}},$ wherein c is the velocity of the radiation, k is a bin number of the peak and N′is a number of bins.
 15. The system according to claim 10, wherein the values of S_(i,n) are set to 1when S_(i,n)>0 and to −1 when S_(i,n)≦0.
 16. The method according to claim 2, wherein said calculating a two-dimensional Fourier transform comprises calculating a first Fourier transform of the array S_(i,n) and to calculating a second Fourier transform of the transformed array.
 17. The system according to claim 10, wherein said processor is configured to calculate a first Fourier transform of the array S_(i,n) and to calculate a second Fourier transform of the transformed array.
 18. A method for determining a respective electromagnetic parameter and spatial disposition of one or more signal sources in a surveillance space simultaneously bombarded by multiple signals, the method comprising: receiving said multiple signals at each of a plurality of antennas that are spaced apart in a linear array by predefined spacings; simultaneously sampling the respective signals of each of the antennas at a predetermined sampling rate to generate a two-dimensional array of sampled values S_(i,n), where S_(i,n) is the n-th sample of the signal received at an antenna i; calculating a two dimensional mathematical transformation over the array S_(i,n) giving rise to a product of the mathematical transformation F_(jk) having a frequency dimension and an azimuth dimension; identifying peaks in the product of the two dimensional mathematical transformation, wherein the identified peaks are peaks that satisfy a predetermined criterion that is indicative of a signal source in the surveillance space; and processing data related to the identified peaks to determine a frequency and an azimuth of the respective signal source, and to determine an amplitude of the signal source wherein the frequency of the respective signal source is calculated using an algebraic involving the predetermined sampling rate. 